Production of difluoromethane

ABSTRACT

A process for the production of difluoromethane comprising (a) contacting dichloromethane with hydrogen fluoride in the presence of a fluorination catalyst to produce a product stream comprising difluoromethane, monochloromonofluoromethane and unreacted starting materials and (b) separating difluoromethane from the product stream from step (a), wherein sufficient hydrogen fluoride is employed in the process such that during step (b) the molar ratio of hydrogen fluoride to monochloromonofluoromethane is at least about 100:1.

This application is a 377 of PCT/GB94/00497 filed Mar. 14, 1994, nowabandoned.

This invention relates to a process for the production ofdifluoromethane.

In recent years there has been increasing international concern thatchlorofluorocarbons, which are used on a large scale around the world,may be damaging the earth's protective ozone layer and there is now inplace international legislation to ensure that their manufacture and useis completely phased out. Chlorofluorocarbons are used, for example, asrefrigerants, as foam blowing agents, as cleaning solvents and aspropellants for aerosol sprays in which the variety of applications isvirtually unlimited. Consequently, much effort is being devoted tofinding suitable replacements for chlorofluorocarbons which will performsatisfactorily in the many applications in which chlorofluorocarbons areused but which will not have the aforementioned damaging effect on theozone layer. One approach in the search for suitable replacements hascentred on fluorocarbons which do not contain chlorine but which containhydrogen. The hydrofluorocarbon difluoromethane, also known as HFA 32,is of interest as one such replacement, in particular in a blend thereofwith other hydrofluoroalkanes, for example HFA 134a and HFA 125, as areplacement for R-22 and R-502 in refrigeration, air-conditioning andother applications.

Processes have been proposed for the production of difluoromethane.Thus, in U.S. Pat. No. 2,744,148, there is described a process for theproduction of difluoromethane comprising contacting dichloromethane withhydrogen fluoride in the presence of a fluorination catalyst whichcomprises nickel, chromium, cobalt, copper or palladium carried onaluminium fluoride. Many other catalysts have been proposed for use inthe hydrofluorination of dichloromethane, for example, chromium fluorideon alumina in U.S. Pat. No. 4,147,733; aluminium fluoride, chromiumfluoride, mixtures thereof, aluminium fluoride on active carbon orferric chloride on active carbon in EP 128510; chromium oxyfluoride inU.S. Pat. No. 2,745,886 and chromia in GB 1,307,224.

However, a serious problem with the production of difluoromethane by thehydrofluorination of dichloromethane is that a substantial amount of ahighly toxic by-product, monochloromonofluoromethane, HCFC 31, isproduced as an intermediate. HCFC 31 has an estimated OccupationalExposure Limit of 10 parts per billion, and may be produced insubstantial quantities, indeed as much as 20% or more of the productfrom the hydrofluorination of dichloromethane.

We have now found that, rather than following the obvious course infinding a solution to this problem, that is to search for conditionsunder which the production of HCFC 31 is reduced, the problem may besolved by suppressing the toxicity problems associated with the HCFC 31which is produced.

According to the present invention there is provided a process for theproduction of difluoromethane comprising: (a) contacting dichloromethanewith hydrogen fluoride in the presence of a fluorination catalyst toproduce a product stream comprising difluoromethane,monochloromonofluoromethane and unreacted starting material and (b)separating difluoromethane from the product stream from step (a),wherein sufficient hydrogen fluoride is employed in the process suchthat during step (b) the molar ratio of hydrogen fluoride tomonochloromonofluoromethane is at least about 100:1.

Preferably during step (b) the molar ratio of hydrogen fluoride tomonochloromonofluoromethane is at least about 150:1, more preferably atleast about 200:1 and especially at least about 300:1.

The Occupational Exposure Limit (O.E.L.) for HCFC 31 is estimated at 10parts per billion, whilst that for hydrogen fluoride is 3 parts permillion. Whilst hydrogen fluoride is therefore toxic, it is estimated tobe about 300 times less toxic than HCFC 31. Furthermore, the toxicityproblems associated with hydrogen fluoride usually exist in reactions inwhich it is employed as the reagent, and in particular where it isemployed in hydrofluorination reactions. Thus, it has been the toxicityof hydrogen fluoride which has determined the safety requirements andthus costs associated with conventional hydrofluorination reactions.

The high toxicity of HCFC 31 however, produced in substantial quantitiesduring the hydrofluorination of dichloromethane would exceed thetoxicity of hydrogen fluoride in the process streams, thereby increasingthe safety requirements, for example the use of specialist highsensitivity equipment for detecting very low levels of HCFC 31, and thuscosts of carrying out the hydrofluorination of dichloromethane.

We have found that where hydrogen fluoride is employed in sufficientquantity, then it may effectively reduce the problem of HCFC 31 byenhancing conversion of HCFC 31 to HFA 32 and at the same time dilutingthe HCFC 31 to concentrations below 30 ppb such that the highestconcentration of HCFC 31 at any point in the process is less than 30ppb, preferably less than 10 ppb and especially less than 3 ppb. In thisway, the predominant toxicity problem to be faced and monitored is thatof hydrogen fluoride. Consequently, the equipment or plant in which theprocess is effected may be operated safely with respect to both hydrogenfluoride and HCFC 31 when it is provided with a system for monitoringand detecting levels of hydrogen fluoride below 5 parts per million.

In particular, the concentration of HCFC 31 may tend to increase duringthe separation of difluoromethane from the process stream, step (b) ofthe process, thereby causing a localised high concentration of HCFC 31.However, hydrogen fluoride remains with the HCFC 31 and thus ifsufficient hydrogen fluoride is present in step (b), then this localisedconcentration of HCFC 31 may be maintained at an acceptable level.

Typically the separation step (b) is performed using distillation anddifluoromethane and hydrogen chloride are recovered from the bottom ofthe distillation column whilst excess hydrogen fluoride, HCFC 31, andunreacted dichloromethane are obtained from the top of the column andrecycled.

According to a preferred embodiment of the present invention there isprovided a process for the production of difluoromethane comprising: (a)contacting dichloromethane with hydrogen fluoride in the presence of afluorination catalyst to produce a product stream comprisingdifluoromethane, monochloromonofluoromethane and unreacted startingmaterials, (b) separating difluoromethane from the product stream fromstep (a) and (c) recovering difluoromethane and recycling HCFC 31 tostep (a) wherein sufficient hydrogen fluoride is employed in the processsuch that during step (b) the molar ratio of hydrogen fluoride tomonochloromonofluoromethane is at least about 100:1.

Usually, where sufficient hydrogen fluoride is employed such that theratio of hydrogen fluoride to monochloromonofluoromethane is at least100:1 in step (b), the ratio of hydrogen fluoride to HCFC 31 will alsobe at least 100:1 for step (c) and step (a).

The amount of hydrogen fluoride which is required in order to achievethe required ratio of hydrogen fluoride to HCFC 31 will depend upon theconversion of dichloromethane and the selectivity to HCFC 31 and HFA 32,that is the amount of HCFC 31 produced in step (a) of the process, whichdepends inter alia upon the conditions of temperature and pressure underwhich the process is operated and the choice of catalyst.

Furthermore, it is not essential that all the hydrogen fluoride ispassed over the catalyst. Thus additional hydrogen fluoride may, asrequired, be added to the process stream recovered from step (a) of theprocess in order to ensure that the required ratio of hydrogen fluorideto HCFC 31 is achieved in step (b). Preferably however, the process isprovided with a single hydrogen fluoride feed to step (a).

Overall, the molar ratio of hydrogen fluoride to dichloromethane whichis fed to the process will be at least 5:1, and usually more than 10:1.There is generally no need to use more than about 100:1 and the molarratio of hydrogen fluoride to dichloromethane will usually be less thanabout 50:1. Where the yield of HCFC 31 in step (a) of the process isabout 10%, the ratio of hydrogen fluoride to dichloromethane may be atleast 10:1, whilst where the yield of HCFC 31 is step (a) is 5%, theratio of hydrogen fluoride to dichloromethane may be at least 5:1.Typically however, the yield of HCFC 31 may be as much as 15%.

The process is preferably operated on a continuous basis, with make-uphydrogen fluoride being fed to step (a) of the process and recycled HCFC31 and unreacted dichloromethane being converted to HFA 32 in step (a)of the process.

The conditions of temperature and pressure and choice of catalystemployed in step (a) may be as described in the prior art, for example atemperature in the range from about 100° C. to about 500° C., preferablyfrom about 200° C. to about 400° C. Atmospheric pressure may beemployed, although superatmospheric pressure, say up to about 30 bar, orsubatmospheric pressures may be employed if desired. The catalyst may bea conventional fluorination catalyst, for example a catalyst based onchromia, chromium fluoride or chromium oxyfluoride, alumina, aluminiumfluoride or aluminium oxyfluoride, or a catalyst comprising a metal, forexample nickel, cobalt, zinc, iron or copper supported upon chromia,magnesia and/or alumina.

We have further found that a fluorination catalyst comprising zinc or acompound of zinc and a metal oxide, fluoride or oxyfluoride may beemployed to increase the selectivity of the process towardsdifluoromethane with a consequent decrease in the yield of HCFC 31 fromthe process. The increased selectivity to difluoromethane provides asubstantial benefit in reducing the levels of HCFC 31 produced, and thusallows less hydrogen fluoride to be employed whilst providing the molarratio of hydrogen fluoride to HCFC 31 required by the present invention.

We prefer to employ a catalyst as described in one of EP 0502605 orPCT/GB93/00244, the disclosures of which are incorporated herein byreference.

Thus the metal of the metal oxide, fluoride or oxyfluoride, the amountof zinc, the catalyst preparation method, the catalyst prefluorinationtreatment, the form of the catalyst, catalyst regeneration treatment,and the presence of other metals or compounds thereof in the catalystmay be as described for the catalysts in EP 0502605 or PCT/GB93/0024,the disclosures of which are incorporated herein by reference. Weespecially prefer a catalyst as described in EP 0502605.

Use of the preferred catalyst generally allows lower temperatures to beused than those employed in the prior art whilst the level of HCFC 31produced may not be increased compared with the levels of HCFC 31produced at higher temperatures using catalysts previously proposed. Theuse of lower temperatures results in substantially longer catalystlifetimes with a consequent reduction in the frequency with which thecatalyst requires regeneration. The temperature is especially preferablyin the range from about 170° C. to about 340° C., and particularly inthe range from about 240° C. to about 320° C.

The invention is illustrated but not limited by the following example.

EXAMPLE

10 g of a zinc/chromium mixed oxide catalyst prepared byco-precipitation and comprising 8% by weight zinc was charged to a 1/2"diameter Inconel reactor tube and heated to 300° C. in nitrogen.Hydrogen fluoride was then passed over the catalyst for 24 hours at 300°C. and the reactor was then cooled to 250° C.

The reactor was pressurised to 10 bar in nitrogen, and dichloromethaneand hydrogen fluoride were passed over the catalyst in the mole ratiosindicated in Table 1. The vent gas from the reactor was scrubbed withwater to remove hydrogen fluoride and hydrogen chloride, sampled andanalysed by Gas Chromatography. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                   Off Gas           HF:CH.sub.2 FCl                                  HF:CH.sub.2 Cl.sub.2                                                                     Composition (% v/v)                                                                             Off Gas                                          (Mole ratio)                                                                             CH.sub.2 Cl.sub.2                                                                     CH.sub.2 ClF                                                                            CH.sub.2 F.sub.2                                                                    Mole Ratio                                 ______________________________________                                        27.1       1.0     7.1       92.0  391                                        21.3       2.3     10.0      87.7  213                                        19.6       2.8     11.1      86.1  175                                        12.5       7.4     9.4       83.1  123                                        ______________________________________                                    

EXAMPLE 2 and 3

The procedure of example 1 was repeated except that the examples wereperformed at atmospheric pressure and the temperature and feed ratio ofhydrogen fluoride to dichloromethane were as stated in Table 2 below.The results are also shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                        Off Gas           HF:CH.sub.2 FCl                             HF:CH.sub.2 Cl.sub.2                                                                  Temp    Composition (% v/v)                                                                             Off-Gas                                     (Mole ratio)                                                                          (°C.)                                                                          CH.sub.2 Cl.sub.2                                                                     CH.sub.2 ClF                                                                          CH.sub.2 F.sub.2                                                                    Mole Ratio                              ______________________________________                                        (Example 2)                                                                   15.9    250     3.5     5.2     91.3  271                                     (Example 3)                                                                   16.1    200     34.7    11.3    54.0  134                                     ______________________________________                                    

EXAMPLE 4 to 7

The procedure of examples 2 and 3 was repeated except that the catalystsemployed were as follows:

Example 4: Chromia having a surface area of 160 m² /g.

Example 5: 2% w/w zinc on alumina prepared by impregnating gamma aluminahaving an initial surface area of 180 m² /g with aqueous zinc chloridesolution.

Examples 6 and 7: 2% w/w chromium on alumina prepared by impregnatinggamma alumina having an initial surface area of 180 m² /g with aqueouschromium chloride solution.

The conditions and results for example 4 to 7 are shown in Table 3below.

                  TABLE 3                                                         ______________________________________                                                        Off Gas           HF:CH.sub.2 FCl                             HF:CH.sub.2 Cl.sub.2                                                                  Temp    Composition (% v/v)                                                                             Off-Gas                                     (Mole ratio)                                                                          (°C.)                                                                          CH.sub.2 Cl.sub.2                                                                     CH.sub.2 ClF                                                                          CH.sub.2 F.sub.2                                                                    Mole Ratio                              ______________________________________                                        (Example 4)                                                                   15.9    200     75.8    14.0    10.2  105                                     (Example 5)                                                                   16.9    200     64.1    15.6    20.3  107                                     (Examples 6                                                                   and 7)                                                                        12.8    200     89.5    8.7     1.8   149                                     15.2    200     91.8    7.0     1.2   218                                     ______________________________________                                    

We claim:
 1. A process for the production of difluoromethane comprising(a) contacting dichloromethane with hydrogen fluoride in the presence ofa fluorination catalyst to produce a product stream comprisingdifluoromethane, monochloromonofluoromethane and unreacted startingmaterials and (b) separating difluoromethane from the product streamfrom step (a), wherein sufficient hydrogen fluoride is employed in theprocess such that during step (b) the molar ratio of hydrogen fluorideto monochloromonofluoromethane is at least about 100:1.
 2. A process asclaimed in claim 1 in which the molar ratio of hydrogen fluoride tomonochloromonofluoromethane is at least about 150:1.
 3. A process asclaimed in claim 1 in which additional hydrogen fluoride is added to theprocess stream recovered from step (a) in order to ensure that therequired ratio of hydrogen fluoride to HCFC 31 is achieved during step(b).
 4. A process for the production of difluoromethane comprising (a)contacting dichloromethane with hydrogen fluoride in the presence of afluorination catalyst to produce a product stream comprisingdifluoromethane, monochloromonofluoromethane and unreacted startingmaterials, (b) separating difluoromethane from the product stream fromstep (a) and (c) recovering difluoromethane and recycling HCFC 31 tostep (a) wherein sufficient hydrogen fluoride is employed in the processsuch that during step (b) the molar ratio of hydrogen fluoride tomonochloromonofluoromethane is at least about 100:1.
 5. A process asclaimed in any one of claims 1 to 4 in which the separation step (b)comprises distilling the product stream from step (a) whereby toseparate a bottom stream comprising difluoromethane and hydrogenchloride from a top stream comprising hydrogen fluoride, HCFC 31 andunreacted dichloromethane.
 6. A process as claimed in any one of claims1 to 5 in which the fluorination catalyst comprises a metal oxide, metalfluoride or oxyfluoride.
 7. A process as claimed in claim 6 in which themetal of the oxide, fluoride or oxyfluoride is at least one of chromium,aluminium, zinc, nickel, cobalt, copper and magnesium.
 8. A process asclaimed in claim 7 in which the catalyst comprises zinc or a compound ofzinc and a metal oxide, fluoride or oxyfluoride in which the metal ofthe oxide, fluoride or oxyfluoride is chromium or aluminium.
 9. Aprocess as claimed in any one of claims 1 to 8 when carried in equipmentprovided with a system for monitoring and detecting concentrations ofhydrogen fluoride below 5 parts per million.
 10. A process as claimed inclaim 9 in which step (a) is effected at a temperature in the range fromabout 240° C. to about 320° C.